Horseshoe Crab

Characterized by its big protective shell, the horseshoe crab is a great example of a living fossil.

One of the oldest animals on earth, horseshoe crabs have been roaming the seas for hundreds of millions of years. Indeed, they have had to adapt to continents forming and reforming and have survived Ice Ages and whatever it was that made the dinosaurs and so many other species go extinct. When humans arrived they discovered that horseshoe crabs could be used as food and as bait to catch other kinds of food, and now they are used for medical research.

Five Incredible Horseshoe Crab Facts!

Some fascinating facts about the horseshoe crab include:

• These animal are arthropods, which means it’s related to spiders.
• They’re not crabs. They’re not even crustaceans.
• They evolved about 255 million years ago, and some consider them living fossils.
• The blood of Carcinoscorpius rotundicauda contains tetrodotoxin, a deadly poison that attacks the nervous system.
• Speaking of blood, the blood of these animals is sapphire blue when exposed to oxygen.

Horseshoe Crab Classification and Scientific name

These animals belong to the Limulidae family as well as the suborder Xiphosurida and the order Xiphosura. There are four genera: Carcinoscorpius; Limulus; Mesolimulus, which is extinct, and Tachypleus. There are four living species in these genera. They are:

• Carcinoscorpius rotundicauda
• Limulus polyphemus
• Tachypleus gigas
• Tachypleus tridentatus

The name Lumulidae is modern Latin for “a little askance,” because it used to be thought the animal had only one eye. Xiphosurida is from the Greek words for sword, “xiphos” and the Greek word for tail, “oura.” Carcinoscorpius means “crab scorpion,” with Carcino coming from the Greek word “karkinos,” a crab, and “scorpius” from the Latin word “scorpius,” which borrowed from the Greek word “skorpios.” Rotundicauda means “round tail” These names were inspired by the look of the animal. The “meso” in mesolimulus is Greek for “middle.” Tachypleus comes from the Greek words “tachys,” which means “rapid” and “pleustikos,” which means “sailing” or “swimming.”

Horseshoe Crab Species

The four horseshoe crab are Carcinoscorpius rotundicauda, which is the mangrove horseshoe crab. This crab is found in southern Asia. Limulus polyphemus is the Atlantic horseshoe crab and it lives along the east coast of the United States and Mexico. Tachypleus gigas is the Indo-Pacific horseshoe crab, and it is also found in southern and southeastern Asia. Tachypleus tridentatus is the tri-spine horseshoe crab, and it lives in southeast Asia and East Asia.

Horseshoe Crab Appearance

The animal’s appearance is unmistakable. The soft parts of its body are completely covered with a tough, horseshoe-shaped carapace. The body has three parts to it. The prosoma can be thought of as the head, then there’s a middle part called the opisthosoma and the tail, or telson. The tail, which is long and stiff, looks menacing, but it’s there to act as a rudder when the horseshoe crab swims and to help turn it upright if it’s turned over.

The females of all species are larger than the males. The male mangrove horseshoe crab is about 12 inches long, but the female of the tri-spine horseshoe crab can be 31.25 inches long and weigh as much as 9 pounds.

The animal has many eyes. It has eyes on and even under its head, and there are even eyes under its tail. This may be why they are sometimes found swimming upside down.

If the animal is flipped upside down, a person will be able to see its book gills, which it uses to breathe underwater. These gills get their name because they look like the pages of a book.

Though they lack teeth, they can still eat shellfish whose shells aren’t too thick. This is because they have a mouth that’s filled with bristles found in the middle of their five pairs of legs. They use their legs to move their food to their mouth, then chew it by walking.

Because their carapace is rigid, the animal needs to molt to grow. They molt frequently during their first year and can molt as many as 18 times before they are of reproductive age, which is about nine. Since females are bigger, they molt more times than males. When the male molts for the last time, he develops claspers that look like boxing gloves. This helps him hold on to the female during the spawning season.

Horseshoe Crab Distribution, Population, and Habitat

All species like shallow water with muddy or sandy bottoms. There are at least millions of these animals in the oceans of the world, though in some places their numbers are declining. The mangrove and tri-spine horseshoe crabs can be found in salt or brackish water around southeast Asia and north into China and Hong Kong. The Atlantic horseshoe crab is found along North America’s Atlantic coastline and in the Gulf of Mexico. The Indo-Pacific horseshoe crab is found on the coasts of the South China Sea down to the Bay of Bengal.

Horseshoe Crab Predators and Prey

These animals eat small fish, bivalves, and other invertebrates. They also eat algae. Predators on young and adult horseshoe crabs include sea turtles, but the greatest contribution they make to the food cycle is their eggs. Horseshoe crabs come to the beach to lay eggs at the same time many shorebirds are migrating. Horseshoe crab eggs are their main source of protein. Fish such as eels, perches, and flounders also eat the eggs and the newly hatched animals. “Real” crabs also indulge in the bounty of eggs.

These animals also has its share of symbionts, since at one point it will no longer molt. Clams scuds, marine worms, algae, barnacles, oysters, starfish, and other creatures live on and in the horseshoe crab’s shell.

For a complete analysis of what horseshoe crabs eat, make sure to read ‘What Do Horseshoe Crabs Eat? The Main Foods in their Diet.‘

Horseshoe Crab Reproduction and Lifespan

These animals can live as long as 20 years, and some biologists believe the animal can live as long as 30 years. Its spawning is one of the most amazing spectacles in nature. They spawn in the spring at high tide during the new or full moon. Males get to the beach first and wait for the females. When a female arrives, a male attaches himself to the back of her shell with his claspers. There may be half a dozen males clinging or attempting to cling to her.

The female carries between 60,000 to 120,000 eggs that she lays about 4000 at a time during high tide. As she lays the eggs, the male fertilizes them. They hatch after about two weeks if the birds and other animals haven’t gorged on them. In places where farming of the horseshoe crab takes place, eggs can be artificially inseminated.

Horseshoe Crab in Fishing and Cooking

These animals do not have much meat on them, but the meat is considered a delicacy by some who are willing to pay a good price for it. In Thailand, they’re part of a dish called yam khai maeng da. People in Asia also eat the eggs. However, it is crucial to find an edible horseshoe crab, for eating one that has tetrodotoxin can be fatal. Horseshoe crabs are also used as bait for eels, conch, and whelks.

Horseshoe Crab Population

There are millions of Atlantic species, even as its status is near threatened. Half a million are used every year for research and at least a million are used for bait. But the tri-spine species is endangered, and there’s not enough data on the other two species.

1. National Geographic, Available here: https://www.nationalgeographic.com/animals/article/covid-vaccine-needs-horseshoe-crab-blood?loggedin=true
2. Promega Connections, Available here: https://www.promegaconnections.com/evaluating-the-costs-of-endotoxin-testing/
3. Animal Diversity Web, Available here: https://animaldiversity.org/accounts/Limulus_polyphemus/#conservation_status
4. The Horseshoe Crab, Available here: https://horseshoecrab.org/
5. Wikipedia, Available here: https://en.wikipedia.org/wiki/Horseshoe_crab
6. EPA, Available here: https://www.epa.gov/ocean-acidification/understanding-science-ocean-and-coastal-acidification